Ion trap mass spectrometry and apparatus

ABSTRACT

A laser-cooled fluorescence mass spectrometry apparatus includes an ion trap for trapping sample ions, laser-cooled ions, and probe ions therein; a first irradiating device for irradiating the sample ions, the laser-cooled ions, and the probe ions in the ion trap with a first laser beam for cooling the ions; a second irradiating device for irradiating the sample ions, the laser-cooled ions, and the probe ions in the ion trap with a second laser beam for detecting temperature changes in the ions; a detecting device for detecting the temperature changes in the ions; a first ion source for the sample ions; a second ion source for the laser-cooled ions; and a third ion source for the probe ions. The probe ions may be different ions than the laser-cooled ions, or the probe ions may be the same ions as the laser-cooled ions.

BACKGROUND OF THE INVENTION

[0001] Field of the Invention

[0002] The present invention relates to a method and apparatus therefor,for an improved laser-cooling fluorescence spectrometry as one type ofion trapping mass spectrometry.

[0003] Ion trapping mass spectrometry is a widely used method for traceanalysis in environmental analysis and other fields. In the mosttypically utilized method, ions trapped in an ion trap are subjected tomass selection and extracted outside the trap, and these extracted ionsare detected with an ion detector such as an electron multiplier. Thistype of mass spectrometry is still the most widely used method and alarge number of reference works are available (such as Reference 1: R.E. March and R. J. Hughes: “Quadrupole Storage Mass Spectrometry” JohnWiley and Sons (1989)).

[0004] Laser-cooled fluorescence mass spectrometry relating to thisinvention is found in a method disclosed in 1995 (Reference 2: U.S. Pat.No. 5,679,950) as a novel ion trap mass spectrometry. In this method,laser-cooling and sympathetic cooling are utilized and sensitivity whichhas been limited to detection levels of about 100 ions in the iontrapping mass spectrometry of the prior art, is significantly improvedby optical detection of the cooled sample ions so that even single ionscan be detected in-situ. In this method, the sample ions are trapped inthe ion trap after mass analysis and can be measured repeatedly todetect ions in an “in-situ” (or non-destructive) manner.

SUMMARY OF THE INVENTION

[0005] The principle of laser-cooled fluorescence mass spectrometry isbriefly described below.

[0006] Ions trapped in a radio-frequency-quadrupole ion trap possess aharmonic oscillation mode due to the potential of the ion trap (inpseudo-potential approximation). The oscillation of these trapped ionsis known as secular (oscillation) motion. The frequencies of thesesecular oscillations are proportional to the charge of the ions and areinversely proportional to the mass. If the secular frequency can bedetected, then the mass spectrometry of the ions can be performed(Reference 1).

[0007] Firstly, the laser-cooled ions and sample ions are simultaneouslytrapped and cooled. The sample ions are sympathetically cooled by thelaser-cooled ions. Next, an electrical oscillation is applied to makethe sample ions resonate at the secular frequency, so that the sampleions are heated by the resonant oscillation. The sample ions arerepeatedly undergoing Coulomb collision with the laser-cooled ions sothat the sample ions transfer energy to the laser-cooled ions and thelaser-cooled ions are sympathetically heated.

[0008] Increase of the temperature of the laser-cooled ions results insuch effects as change of the fluorescence intensity, and change in thespatial distribution of the fluorescence. These changes give informationon the mass and the amount of the sample ions.

[0009] The mechanism of fluorescence intensity change in thelaser-cooled fluorescence mass spectrometry can be understood by thefollowing brief theoretical analysis.

[0010] First, using a simplified theory of Doppler laser-cooling, therelation of laser cooling efficiency and fluorescence intensity iscalculated with respect to ion temperature.

[0011] When a laser beam with a fixed wave vector irradiates free atoms(or ions), a force acts on the atoms in the direction of the wave vectorof the light due to photon scattering. Laser cooling is performed usingthis force. Typically, an atom having a simple two-level transition ischosen to avoid optical pumping, and the laser wavelength is adjusted tothe resonance transition of the two-level atom species.

[0012] When the momentum of the atom is counter to the direction of thewave vector of the laser light, the velocity of the atoms decreasesafter resonant absorption of photons due to momentum transfer of photonsto the atom. Conversely, when the momentum of the atom is in the samedirection of the wave vector of the laser light, then the velocityincreases.

[0013] When the detuning frequency is negative, in other words, when thelight has a wavelength slightly longer than the resonant wavelength, theprobability of resonance scattering increases when the atoms aretraveling counter to the direction of laser light wave vector, comparedto when traveling in the same direction to the light wave vector due tothe Doppler effect. In this case, consequently, the energy of the atomis lost and the atom cools down.

[0014] Spatially-uniform natural emission following absorption resultsin a random-walk increase of energy, whose balance with the coolingeffect determines the ultimate attainable temperature h Γ/2, where Γ isthe natural linewidth of the transition. As shown below, the averageenergy change due to resonant absorption is h Δν, where Δν is thedetuning frequency, which is the deviation of the laser light frequencyfrom the resonant frequency ν₀ of the atoms at rest. When |h Δν| isgreater than h Γ/2, we can neglect the random-walk heating by naturalemission, which is the case considered below to simplify discussion.

[0015] An average energy change h Δν due to resonance absorption ofphotons can be explained by simple kinetics. The wave vector directionof the laser light is set as the z axis. The velocity component alongthe z axis of the atom with mass m is defined as v_(z) in the laboratoryframe. In the center of mass system, the frequency of the light shiftsν(1−v_(z)/C) due to the Doppler effect. When ν(1−v_(z)/C) matches theresonant frequency ν₀, a resonant scattering occurs. The atom obtainsmomentum h ν₀/c from the laser light when the atom absorbs the light.Next, the atom emits photon by spontaneous emission process. This photonemission is uniform in all directions so net average change of the atommomentum does not occur. Consequently, the atom obtains momentum h ν₀/Cby the resonant scattering of the laser light.

[0016] The velocity after the resonant scattering becomes v_(z)′=V₂+hν₀/mc when observed in the laboratory frame. The change ΔE of kineticenergy is $\begin{matrix}\begin{matrix}{{\Delta \quad E} = \quad {\frac{{mv}_{z}^{\prime 2}}{2} - \frac{{mv}_{z}^{2}}{2}}} \\{= \quad {h\quad \Delta \quad {v\left( {1 + \frac{h\quad \Delta \quad v}{4E}} \right)}}} \\{\cong \quad {h\quad \Delta \quad v}}\end{matrix} & \left( {{Equation}\quad 1} \right)\end{matrix}$

[0017] When the atom is in the laser beam, the rate of resonantscattering per unit time is given by $\begin{matrix}{{{sr}\left( {v_{z},I} \right)} = \frac{{\Gamma\Omega}_{rabi}^{2}}{\left( {v - v_{0} - \frac{{vv}_{z}}{c}} \right)^{2} + \Gamma^{2} + \Omega_{rabi}^{2}}} & \left( {{Equation}\quad 2} \right)\end{matrix}$

[0018] Here, Γ is the natural width of the transition. Ω_(Rabi) is theRabi frequency, which depends on the light intensity.

[0019] Using these relations, an atom with velocity v_(z) attains anenergy change per unit time ΔE_(scatt) due to the resonant scattering;$\begin{matrix}{{\left( \frac{\Delta \quad E_{scatt}}{\Delta \quad t} \right)\left( {v_{z},I,{\Delta \quad v}} \right)} = {h\quad \Delta \quad v\quad {{sign}\left( v_{z} \right)}{{sr}\left( {v_{z},I} \right)}}} & \left( {{Equation}\quad 3} \right)\end{matrix}$

[0020] Here, sign (V_(z)) is a symbol that indicates the direction ofthe motion of the atom relative to the laser beam. It is negative whenthe directions of the atom and light beam are opposite, in which casethe atom slows down. It is positive when the directions are the same, inwhich case the atom accelerates.

[0021] Next, the average energy and fluorescence intensity is calculatedwhen laser-cooling is applied to the ions. Here, it is assumed that theions are in a gaseous state (not in a Wigner-crystal state). Thevelocity distribution n (v_(x), v_(y), v_(z)) can be written as aMaxwell distribution with an ion temperature T; $\begin{matrix}{{n\left( {v_{x},v_{y},v_{z}} \right)} = {{N\left( \frac{m}{2\pi \quad {kT}} \right)}^{\frac{3}{2}}{\exp\left( {{- m}\frac{v_{x}^{2} + v_{y}^{2} + v_{z}^{2}}{2{kT}}} \right)}}} & \left( {{Equation}\quad 4} \right)\end{matrix}$

[0022] Modifying this equation to one-dimension, the laser coolingefficiency, which is defined as the average energy change of an ion bylaser-cooling per unit time, can be described as $\begin{matrix}{\left( \frac{m}{2\pi \quad {kT}} \right)^{\frac{1}{2}}{\int{\left( \frac{\Delta \quad E_{scatt}}{\Delta \quad t} \right)\left( {v_{z},I,{\Delta \quad v}} \right){\exp \left( {{- m}\frac{v_{z}^{2}}{2{kT}}} \right)}{v_{z}}}}} & \left( {{Equation}\quad 5} \right)\end{matrix}$

[0023] The fluorescence intensity per unit time of an ion withtemperature T, averaged over the velocity distribution, is$\begin{matrix}{\left( \frac{m}{2\pi \quad {kT}} \right)^{\frac{1}{2}}{\int{{{sr}\left( {I,v_{z}} \right)}{\exp \left( {{- m}\frac{v_{z}^{2}}{2{kT}}} \right)}{v_{z}}}}} & \left( {{Equation}\quad 6} \right)\end{matrix}$

[0024] In laser-cooled fluorescence mass spectrometry, analternating-current electrical field is applied to excite the secularmotion of the sample ions. The maximum heating rate by this AC electricfield, which is defined as the maximum value of the energy increaseΔE_(heat) per unit time, can be described as $\begin{matrix}{\left( \frac{\Delta \quad E_{heat}}{\Delta \quad t} \right)_{\max} = {\left( \frac{2{kT}}{\pi \quad m} \right)^{\frac{1}{2}}\frac{e\quad V_{a\quad c}}{8r_{0}}}} & \left( {{Equation}\quad 7} \right)\end{matrix}$

[0025] where T is the ion temperature, V_(ac) is the voltage amplitudeof dipole AC electrical field applied to the electrodes. Hereafter,Equation 7 is derived.

[0026] A precise calculation of the actual heating rate (or the changeof temperature) of the ions might not be simple because the Coulombinteraction between ions constitutes a many-body non-linear system, andbecause a precise calculation must include laser-cooling effect at thesame time, which also depends non-linearly on the ion velocity. On theother hand, to determine a suitable set of operation parameters for thelaser-cooled fluorescence mass spectroscopy, an approximate knowledge ofthe maximum heating rate in the absence of laser-cooling is quite usefulas will be discussed later. Thus, we attempt to find the maximum upperlimit for the heating rate under the following conditions:

[0027] (a) there is no laser cooling,

[0028] (b) applied AC oscillation frequency ω matches the secular motionoscillation ω₀, and

[0029] (c) Coulomb interaction between ions is ignored.

[0030] That is, heating due to the trapping radio-frequency is ignored,since it is much smaller than the resonant forced-oscillation heating.

[0031] Further, we assume the equation of motion as a one-dimensionalforced oscillation without damping: $\begin{matrix}{\frac{^{2}x}{t^{2}} = {{\frac{f}{m}{\exp \left( {{\omega}\quad t} \right)}} - {\omega_{0}^{2}x}}} & \left( {{Equation}\quad 8} \right)\end{matrix}$

[0032] where f is the amplitude of the force. In the present case, ACelectric voltage V_(ac) is applied to two pairs of adjacent quadrupoleelectrodes, resulting in an approximate oscillating electric fieldamplitude f=eV_(ac)/(2r₀)

[0033] When Equation 8 is solved for velocity v with initial conditionsv(t=0)=0 and x(t=0)=0, $\begin{matrix}{v = {{\frac{f\quad t}{2m}{\cos \left( {\omega_{0}t} \right)}} = {v_{0}{\cos \left( {\omega_{0}t} \right)}}}} & \left( {{Equation}\quad 9} \right)\end{matrix}$

[0034] where v₀=ft/2m.

[0035] The energy E at time t is given by $\begin{matrix}{E = {{\frac{1}{4}{mv}_{0}^{2}} = {\frac{f^{2}}{16m}t^{2}}}} & \left( {{Equation}\quad 10} \right)\end{matrix}$

[0036] Extending this result, we approximate the heating rate of any ionat velocity V₀ by $\begin{matrix}{\frac{E}{t} = {{\frac{f^{2}}{8m}t} = {\frac{f}{4}v_{0}}}} & \left( {{Equation}\quad 11} \right)\end{matrix}$

[0037] Assuming a Maxwellian distribution of ion velocity V₀ withtemperature T, the maximum heating rate is approximated by$\begin{matrix}\begin{matrix}{\frac{\Delta \quad E_{heat}}{\Delta \quad t_{\max}} = \quad {\left( \frac{m}{2\pi \quad k\quad T} \right)^{\frac{1}{2}}{\int{\frac{E}{t}{\exp \left( \frac{{- m}\quad v_{0}^{2}}{2k\quad T} \right)}{v_{0}}}}}} \\{= \quad {\left( \frac{2k\quad T}{\pi \quad m} \right)^{\frac{1}{2}}\frac{e\quad V_{ac}}{8r_{0}}}}\end{matrix} & \left( {{Equation}\quad 12} \right)\end{matrix}$

[0038]FIG. 4 through FIG. 8 show calculated results of the temperaturedependence of the laser cooling rate (or, cooling efficiency) of atypical laser-cooled ion ²⁴Mg⁺ at various laser beam parameters. Eachfigure includes curves for various values of the detuning frequency. Thehorizontal axis shows the ion temperature. The vertical axis shows thelaser-cooling rate. The laser-cooling rate has a maximum cooling rate attemperatures between about 1 and 100 K.

[0039] The figures also show the temperature dependence of the maximumheating rate at various values of analysis voltage V_(ac). The verticalaxis shows the maximum heating rate. The laser beam, which is focused toa diameter of 0.2 mm, has a power of 1 μW, 10 μW, 100 μW, 1 mW and 10mW, respectively in each figure.

[0040]FIG. 9 through FIG. 13 show calculations of the temperaturedependence of the fluorescence intensity of ²⁴Mg⁺ ions at various laserbeam parameters. Each figure includes curves for various values of thedetuning frequency. The horizontal axis shows the ion temperature. Thevertical axis shows the fluorescence intensity. The figures respectivelyshows results for laser beams of 1 μW, 10 μW, 100 μW, 1 mW and 10 mWfocused to a diameter of 0.2 mm.

[0041]FIG. 14 through FIG. 17 show calculated results of the temperaturedependence of the laser cooling rate of a typical laser-cooled ion¹³⁸Ba⁺ at various laser beam parameters. The figures also show thetemperature dependence of the maximum heating rate at various values ofanalysis voltage V_(ac). FIG. 18 through FIG. 21 show calculations ofthe temperature dependence of the fluorescence intensity of ¹³⁸Ba⁺ ionsat various laser beam parameters.

[0042] The following data are used in the calculations. Mass of ²⁴Mg⁺ m= 24 m_(u) Resonance wavelength of ²⁴Mg⁺ λ₀ = 280 nm Natural width of²⁴Mg⁺ Γ = 43 MHz Mass of ¹³⁸Ba⁺ m = 138 m_(u) Resonant wavelength of¹³⁸Ba⁺ −λ₀ = 493 nm Natural width of ¹³⁸Ba⁺ Γ = 15.1 MHz

[0043] In the calculations of ¹³⁸Ba⁺, it is treated as a 2-level atom,where the pump-back transition is ignored, and only the laser-coolingtransition of ¹³⁸Ba⁺ is taken into account.

[0044] The above calculations give insight on the mechanism of how themass-signal is produced in laser-cooled fluorescence mass spectrometry,and teaches us a valuable guidance on how to stably obtain the signal.

[0045] Firstly, the mechanism of the signal generation is consideredfrom the relation between ion temperature and fluorescence intensity.

[0046] We consider the case of Ω_(Rabi)≦Γ, where the resonancescattering is not strongly saturated. At detuning frequency smaller thannatural width of the transition, the fluorescence intensity increaseswhen the ions are laser-cooled and drop its temperature. (See forinstance, the characteristic for a detuning frequency from 0 to 40 MHzin FIG. 9.). We observe this typical effect when laser coolingexperiments are performed. When the detuning frequency becomes muchlarger than the natural linewidth, the fluorescence intensity reaches anmaximum value at temperature around one to ten Kelvin (See for instancethe characteristic in FIG. 9 when the detuning frequency is larger than−50 MHz.) This maximum occurs because the probability of absorbing laserlight becomes larger with a wider velocity distribution due to theDoppler effect.

[0047] Next, we explain the case of Ω_(Rabi)>>Γ, where the resonancescattering is strongly saturated. Though a strong fluorescence intensitycould be obtained by causing saturation, the dependence of fluorescenceintensity on ion temperature and detuning frequency became smaller (Seefor instance, the characteristic of FIG. 12 and FIG. 13.). This effectappears because the width of resonant absorption spectra widens due tosaturation, so that the resonance scattering rate does not depend somuch on the ion velocity at small detuning frequencies.

[0048] In laser-cooled fluorescence mass spectrometry, information onchanges of temperature is obtained from the changes in fluorescenceintensity. For maximum signal, it is necessary to maximize the change offluorescence. Above arguments teach us that, to this end, it isdesirable to keep the laser power low enough not to saturate thetransition, and to keep the detuning to zero.

[0049] Next, we discuss the conditions for maintaining non-destructiveanalysis using the relation between ion temperature and laser coolingrate.

[0050] In our calculation, the laser-cooling rate has a maximum coolingrate at temperatures between about 0.1 K and 100 K when Ω_(Rabi)<Γ. Atlower temperature, the cooling rate decreases as the temperaturedecreases. The laser cooling rate approaches zero as the temperaturedecreases, because, in our approximation, the decrease of the width ofthe Doppler velocity distribution results in the decrease of mean energyloss due to photon absorption. In reality, as the temperature approacheszero, the heating effect from natural emission of photons by excitedions must be taken into account, whose balance with the cooling effectby photon absorption will determine the lowest temperature attainable.Since the temperature in our calculation is much higher than thetemperature where such a heating by natural emission becomes important,we considered only the cooling effect by absorption.

[0051] Above the maximum cooling rate, the rate drops as the temperaturerises. At higher temperatures, the laser cooling rate decreases due todecreased probability of photon absorption.

[0052] We now describe ion stability in laser-cooled fluorescence massspectrometry, using these calculation of the laser cooling rate and themaximum heating rate by the forced oscillation. Ion loss may occur whenthe maximum heating rate is larger than the laser cooling rate.

[0053] When the sample ions are oscillated by external fields, theheating rate is proportional to the number of sample ions, and thecooling rate is proportional to the number of laser-cooled ions. Theheat coupling between the laser-cooled ions and the sample ions dependson the sympathetic cooling rate. In the following typical calculation,the number of ions is set equal to the number of laser-cooled ions, andthe calculation assumes that the sympathetic coupling is complete, i.e.,that there is no temperature difference between the sample ions and thelaser-cooled ions.

[0054] When the maximum heating rate from heating (hereafter, analysisheating) due to the forced oscillation of the ion by the analysisvoltage is smaller than the laser cooling rate, ions are not lost due toanalysis, and are stable. At a fixed set of values of theanalysis-voltage amplitude and laser beam parameters, the intersectionof the maximum-heating-rate line and the laser-cooling curve provides aguide for establishing the temperature where the cooling and heating arebalanced. Of the two intersections that may exist, the intersection onthe lower temperature side provides a guide for the ion temperatureduring analysis heating. If the heating rate due to analysis heating atthis ion temperature is increased by increasing the analysis voltage,the ion temperature shifts to a higher value. When the analysis voltageis further increased, the maximum heating rate line and the lasercooling line come in contact at a single point. See, for instance, thecontact made by the line for analysis voltage 0.9 mV in FIG. 4 and theline for the detuning frequency −30 MHz. This point yields the maximumtemperature that can be stably reached under that specific laser coolingconditions. When a larger analysis voltage is applied to provide furtherheating, the analysis heating rate exceeds the laser cooling rate, sothat the ions are heated at any temperature without reaching a stablebalance with cooling, and ions are lost. That is, an upper limit foranalysis voltage exists at a fixed laser-cooling condition whereanalysis is performed without losing ions. Hereafter, this temperatureis referred to as the maximum ion temperature, and is shown by the whitecircles in the FIGS. 4-8, FIGS. 14-17, FIG. 22, and FIG. 23.

[0055] Therefore, in laser-cooled fluorescence mass spectrometry asshown above, by applying cooling that is stronger than the appliedforced-oscillation, the laser-cooled ions and sample ions can be trappedstably in the ion trap. Preferably, parameters should be chosen so thatstrong and stable cooling is realized over as wide an ion temperaturerange as possible in the presence of temperature increase due toanalysis heating. The present calculations show that, for effective andstable cooling, the intensity of the laser beam should preferably be sostrong as to saturate the transition, and the laser detuning shouldpreferably be much larger than the natural linewidth of the transition.The extent of saturation of the intensity should not be so strong as tolowering the laser-cooling efficiency itself.

[0056] We point out two issues in the laser-cooled fluorescence massspectrometry of the prior art. One issue is that finding the lasercooling conditions for obtaining a signal is difficult in general. Forinstance, when the number of ions changes, then new conditions must besearched for. Another issue is that even if conditions are found forobtaining a signal, loss of ions in the trap frequently occurs due toanalysis heating.

[0057] As shown before, in-situ (in-trap) analysis can be performed inprinciple in the spectrometry of the prior art. However, to make in-trapanalysis possible, a plurality of strict optimal parameters arerequired. If optimal conditions are not provided, then the ions arelost, or no signal can be obtained. Since selecting the correct analysisparameters is difficult, considerable experience is needed to implementthe spectrometry of the prior art.

[0058] The intent of this invention is to overcome these two issues ofthe prior art. In the prior art, these two issues arise because onespecies of ions is simultaneously used both as a laser-cooled coolantmeans and as an ion temperature probe means. In the prior art, a largedetuning frequency (preferably much larger than the natural linewidth)is required to obtain sufficient laser-cooling efficiency over a widetemperature range. On the other hand, small detuning frequency(preferably smaller than the natural linewidth) is required to obtain astrong signal intensity (change of fluorescence intensity). These twoconflicting conditions are the cause of the issues of the prior art.This present invention resolves these mutually conflicting conditions,by providing independent and isolated means for the laser-coolingcoolant and the ion-temperature probe.

[0059] Hereafter, a detailed description of the invention is given.First of all, a method for effectively selecting the laser coolingconditions is explained.

[0060] In order to increase the change of the fluorescence intensity ofthe ion-temperature probe after analysis heating, ion temperature beforeanalysis should preferably be set to a low temperature below 1 K.However, if the temperature is too low (below 0.1 K), ioncrystallization may occur, so that the oscillation frequency may differfrom the secular motion frequency, which complicates data analysis. Tosimplify the analysis, a gaseous phase of ion should be preferablyachieved by a balance between the laser cooling and heating, for whichheating, typically, a trapping-radio-frequency heating effect isdominant in the absence of analysis heating. A large detuning frequencyof −100 MHz or more, which is much larger than the natural linewidth ofthe transitions presently used, is utilized for the laser-cooling beamto achieve a gaseous phase within temperature 0.5 K to 1K. The laserintensity is set to a saturated intensity with saturation parameterΩ_(Rabi)/Γ≈1 to 5. For effective probing, the laser beam for the probelight should preferably have a detuning frequency much smaller than thenatural linewidth, preferably in the vicinity of 0 MHz (Δν=−20 MHz to 20MHz: optimized at 0 MHz), and its intensity should preferably set belowthe saturation intensity (saturation parameter: from approximatelyΩ_(Rabi)/Γ=0.1 to 1).

[0061] Two methods for separating the laser cooling means and the iontemperature probe means are explained next.

[0062] Method (1):

[0063] A method with separate ion species for the laser cooled ions andthe probe ions.

[0064] In this method, separation of the laser cooled means and the ionprobe means are realized by using two ion species each supporting theirrespective function. To obtain strong laser-cooling, one ion species,which can be laser-cooled, is used as the laser-cooled ion using asaturating laser light with a large detuning frequency. To generateprobe fluorescence, another ion species, which is able to belaser-cooled, is used as the probe ion, by utilizing a weak laser lightin the vicinity of the 0 MHz detuning frequency. Effective operation canbe attained within the natural width, typically at approximately Δν=−20MHz to 20 MHz; however, 0 MHz is optimal, so that one can monitor thechanges in fluorescence due to analysis heating of the sample ions.

[0065] It is effective to use two different isotopes of the same elementas the two species of atomic ions for the laser cooled ions and theprobe ions. In such a case, the two fluorescence wavelengths will benearly identical. By using a method such as adding the laser intensitymodulation to the probe light and then extracting the fluorescenceintensity components synchronized with that modulation, only thefluorescence emitted by the probe ions can be monitored. For instance,by generating probe light intermittently with an optical chopper, thefluorescence intensity emitted by the probe ions will be detected onlywhen the probe light is On.

[0066] Method (2):

[0067] A method using a single ion species and two laser beams, one fora laser cooling means and the other for a probing means.

[0068] The separation of the laser cooling means and the probing meanscan be realized by using at least two laser beams to excite singlespecies of laser-coolable ions to be used both as a coolant and a probe.One laser beam is used as a laser cooling means providing a saturatinglaser light at a large detuning frequency. Another laser light is usedas a probe means providing a weak laser light substantially at adetuning frequency of zero (0). Fluorescence generated by the probelight is monitored.

[0069] The laser-cooling beam saturates the cooling transition of theions. This saturation will affect the fluorescence excited by the probebeam. This reduces the ability of the probe beam to detect the iontemperature. Following two methods are effective in avoiding thisdeterioration in the ability to monitor ion temperature. In one method,the energy level used in laser cooling and the energy level used in theprobe are separated. Separating these energy levels means that effectson probe light saturation can be avoided In the other method, thelaser-cooling light and the probe light excite the same level, but thelaser-cooling light is stopped during observation of the fluorescencefrom the probe light. One example of achieving this intensity modulationis to use an optical chopper on the laser-cooling beam, so that onedetects the fluorescence by the probe light when the cooling beam isoff.

BRIEF DESCRIPTION OF THE DRAWINGS

[0070]FIG. 1 is a block diagram showing the concept of the apparatus inone embodiment of this invention, where the laser-cold ions and probeions are separate.

[0071]FIG. 2 is drawing illustrating the concept of the linear ion traputilized in FIG. 1.

[0072]FIG. 3 is a block diagram showing the concept of a typicalapparatus in another embodiment of this invention, where laser-coolinglight and probe light of two wavelengths are applied on one species oflaser-coolable ion.

[0073]FIG. 4 is a graph showing calculated results of the laser-coolingrate and the maximum heating rate provided by an analysis voltage, wherethe laser-cooled ions are magnesium ions, the laser beam is focused to0.2 mm, and the laser beam intensity is 1 μW. Calculations are performedfor detuning frequencies −10, −20, −30, −40, −50, −60, −70, −80, −90,−100, −200, −300, −400, −500, −600, −700, −800, and −900 MHz.

[0074]FIG. 5 is a graph showing calculated results when the laser beamintensity is 10 μW. Other conditions are the same as in FIG. 4.

[0075]FIG. 6 is a graph showing calculated results when the laser beamintensity is 100 μW. Other conditions are the same as in FIG. 4.

[0076]FIG. 7 is a graph showing calculated results when the laser beamintensity is 1 mW. Other conditions are the same as in FIG. 4.

[0077]FIG. 8 is a graph showing calculated results when the laser beamintensity is 10 mW. Other conditions are the same as in FIG. 4.

[0078]FIG. 9 is a graph showing calculated results of the relationbetween fluorescence intensity and ion temperature, where thelaser-cooled ions are magnesium ions, the laser beam is focused to 0.2mm, and the laser beam intensity is 1 μW.

[0079]FIG. 10 is a graph showing calculated results when the laser beamintensity is 10 μW. Other conditions are the same as in FIG. 9.

[0080]FIG. 11 is a graph showing calculated results when the laser beamintensity is 100 μW. Other conditions are the same as in FIG. 9.

[0081]FIG. 12 is a graph showing calculated results when the laser beamintensity is 1 mW. Other conditions are the same as in FIG. 9.

[0082]FIG. 13 is a graph showing calculated results when the laser beamintensity is 10 mW. Other conditions are the same as in FIG. 9.

[0083]FIG. 14 is a graph showing calculated results of the laser-coolingrate and the maximum heating rate provided by an analysis voltage, wherethe laser-cooled ions are barium ions, the laser beam is focused to 0.2mm, and the laser beam intensity is 1 μW.

[0084]FIG. 15 is a graph showing calculated results when the laser beamintensity is 10 μW. Other conditions are the same as in FIG. 14.

[0085]FIG. 16 is a graph showing calculated results when the laser beamintensity is 100 μW. Other conditions are the same as in FIG. 14.

[0086]FIG. 17 is a graph showing calculated results when the laser beamintensity is 1 mW. Other conditions are the same as in FIG. 14.

[0087]FIG. 18 is a graph showing calculated results of the relationbetween fluorescence intensity and ion temperature, where thelaser-cooled ions are barium ions, the laser beam is focused to 0.2 mm,and the laser beam intensity is 1 μW.

[0088]FIG. 19 is a graph showing calculated results when the laser beamintensity is 10 μW. Other conditions are the same as in FIG. 18.

[0089]FIG. 20 is a graph showing calculated results when the laser beamintensity is 100 μW. Other conditions are the same as in FIG. 18.

[0090]FIG. 21 is a graph showing calculated results when the laser beamintensity is 1 mW. Other conditions are the same as in FIG. 18.

[0091]FIG. 22 is a graph illustrating selection of the laser parametersin the first embodiment shown in FIG. 1.

[0092]FIG. 23 is a graph illustrating selection of the laser parametersin the second embodiment shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0093] Embodiment I

[0094]FIG. 1 and FIG. 2 shows a description of an embodiment of thepresent invention, where the laser cooled ions and the probe ions aredifferent as explained above in Method (1) The laser-cooled ion used inthis embodiment is ²⁴Mg⁺, and the probe ion is ²⁶Mg⁺. The naturalabundance ratio of magnesium isotope is approximately24Mg⁺:²⁵Mg⁺:²⁶Mg⁺=8:1:1.

[0095]FIG. 1 is a block diagram illustrating the structure of theapparatus of the embodiment. A radio-frequency-quadrupole linear iontrap shown in FIG. 2, which is placed inside an ultra-high vacuumchamber, is utilized as the ion trap means 6. The linear ion trapcomprises the four electrodes 21 through 24 that have a hyperbolic crosssection. In this embodiment, the electrode group 25 through 28, whichhave equivalent shape as the linear ion trap, is used as an ion sourcesection. The end electrode groups 29 through 32, and 33 through 36 areinstalled at the ends to which direct current voltages 39, 41 areapplied to prevent the ions from leaking out of the electrode ends. Adirect current voltage 40 is further applied to the ion source sectionto prevent ions from leaking out of the ion trap. In this embodiment,the electrode structure on the end section is a linear quadrupolestructure. Ions can be trapped along the axis of the electrodes byapplying a radio-frequency-quadrupole voltage to these electrodes by apower supply 37. A specific example of this ion trap is found in thepreviously mentioned U.S. Pat. No. 5,679,950 relating to the applicationsubmitted by the inventors of the current application, where FIG. 1through FIG. 5. of U.S. Pat. No. 5,679,950 give a detailed explanationand drawings.

[0096] In the following explanation, the electrode diameter of the iontrap electrodes is designated as r₀, When a radio frequency quadrupolevoltage V having a frequency Ω, is applied to the electrodes 21 through24 of the linear ion trap and the electrodes 25 through 28 of the ionsource section, in the absence of DC quadrupole component, the radiofrequency field φ, can be described by; $\begin{matrix}{\varphi = \frac{V\quad {\cos \left( {\Omega \quad t} \right)}\left( {x^{2} - y^{2}} \right)}{2r_{0}^{2}}} & \left( {{Equation}\quad 13} \right)\end{matrix}$

[0097] The equation of motion of the ions within the plane perpendicularto the ion trap axis is; $\begin{matrix}\begin{matrix}{{mx}^{''} = \quad \frac{e\quad V\quad {\cos \left( {\Omega \quad t} \right)}x}{r_{0}^{2}}} \\{{my}^{''} = \quad \frac{{- e}\quad V\quad {\cos \left( {\Omega \quad t} \right)}y}{r_{0}^{2}}}\end{matrix} & \left( {{Equation}\quad 14} \right)\end{matrix}$

[0098] This equation of motion is the Mathieu equation. By using theparameter q, and the normalized time ξ, such that; $\begin{matrix}\begin{matrix}{q = \quad \frac{2e\quad V}{m\quad \Omega^{2}r_{0}^{2}}} \\{\xi = \quad \frac{\Omega \quad t}{2}}\end{matrix} & \left( {{Equation}\quad 15} \right)\end{matrix}$

[0099] We obtain a variation of the standard Mathieu equation;$\begin{matrix}{{{\frac{d^{2}x}{d\quad \xi^{2}} - {2q\quad {\cos \left( {2\xi} \right)}x}} = 0}{{\frac{d^{2}y}{d\quad \xi^{2}} - {2q\quad {\cos \left( {2\xi} \right)}y}} = 0}} & \left( {{Equation}\quad 16} \right)\end{matrix}$

[0100] To discuss the stability of trapped ions, in the absence of DCquadrupole field, the parameter q is used. Stable condition is realizedwhen q is less than 0.908. When q is 0.5 or less, motion of trapped ionscan be approximated by harmonic oscillation known as secular motion.Particularly, when no DC voltage is applied, the equation of motion is;$\begin{matrix}{x^{''} = \frac{{- \Omega^{2}}q^{2}x}{8}} & \left( {{Equation}\quad 17} \right)\end{matrix}$

[0101] This motion is called secular motion, and its frequency ω, whichis called secular frequency is given by; $\begin{matrix}{\omega = \frac{\Omega \quad q}{2\sqrt{2}}} & \left( {{Equation}\quad 18} \right)\end{matrix}$

[0102] In this embodiment, ²⁴Mg⁺ and ²⁶Mg⁺ ions are respectively used asthe laser cooled ions and the probe ions. Conditions of radio frequencyare chosen so as to stably trap these ions at approximately q=0.2.Conditions for stable trapping can be achieved, for example, byapproximately V=40 volts and Ω/2 π=3 MHz when r₀ equals 3 mm.

[0103] As stated previously, by applying a direct current voltage on theion trap electrodes using the direct current power supplies 39 through41, the leakage of ions from the electrode edges can be prevented andions can be accumulated in the ion trap sections 21 through 24.

[0104] Mass-spectrometric analysis can be implemented by measuring thesecular frequency ω of the sample ions in the laser-cooled fluorescencemass spectrometry. An AC voltage of approximately 1 millivolt is appliedbetween two adjacent pairs of linear ion trap electrodes consisting of aset of electrodes 21, 22 and a set of electrodes 23, 24 by utilizing amass-analysis power supply 7 composed of a power supply where thefrequency can be sweeped. Using this power supply voltage, a dipoleelectrical field within the ion trap is created to resonantly oscillatethe sample ions, and forced-oscillation heating of the cooled ions isperformed.

[0105] The respective resonance wavelength of the ²⁴Mg⁺ laser-cooledions and the ²⁶Mg⁺ probe ions utilized in this embodiment is around 280nm. The laser light source 1 in the figure is for laser cooling of the²⁴Mg⁺ ions, which is achieved, for example, by a dye laser pumped by anargon ion laser and equipped with a second harmonic generation crystalsuch as a KDP crystal. The laser light source 3 in the figure is forgenerating probe laser light to excite fluorescence of the ²⁶Mg⁺ probeions, and can be achieved by a laser comprising the same structure asthe laser-cooling laser. The respective wavelengths emitted however aredifferent slightly. Since detailed description of the process of lasercooling of the ²⁴Mg⁺ ions is well known and reported in many researchstudies, we omit its explanation. The reference numerals 2 and 4 arelines showing the laser beam outputs from the respective laser lightsources. Reference numeral 5 is a chopper for making chopping the laserlight 4 periodically. A broken line is shown downstream the chopper 5 inthe figure to signify intermittent emission of the laser light 4. Amechanical optical chopper may be used as the chopper 5, oralternatively, the optical path of the light can be regulated by meansof an acousto-optical modulator to perform intermit control of theincoming light beam to the ion trap means 6. Reference numeral 8 is anphotomultiplier tube and detects the fluorescence of the Mg⁺. Referencenumeral 10 shows ions trapped in the ion trap means 6 such as lasercooled ions, probe ions and sample ions.

[0106] The fluorescence, which is shown by the solid and broken lines inthe figure, from the laser cooling light 2 and from the probe light 4 isdetected by a photomultiplier tube 8. On/off modulation of the probelight 4 is achieved by the optical chopper 5. The fluorescence componentsynchronized with the on/off operation of the optical chopper 5 isextracted by a lock-in detector for photons 9, so that the fluorescenceemitted by the ²⁶Mg⁺ probe ions is extracted out of the total detectedfluorescence. The inset shows a schematic figure of a typical data, withthe frequency of the analysis voltage taken along the horizontal axis,and the fluorescence intensity of the probe ions is taken along thevertical axis.

[0107] Mass-spectrometric analysis starts with the loading of ions intothe trap. In this embodiment. The magnesium vapor enters the ion sourcesections 25 through 28 where the ions are generated by electron impact.A display of atomic oven, atomic beam, the electron beam, electron beampower supply is omitted from the figure. The sample gases are alsointroduced into the ion source section 25 through 28 in a similar mannerand are ionized. Next, ²⁴Mg⁺ is laser-cooled by the laser light 2. Somecooling period is required for the sample ions and the probe ions to besympathetically cooled, according to the effects of the ²⁴Mg⁺ lasercooling. On reaching equilibrium, the fluorescence emitted by ²⁴Mg⁺, andthe fluorescence emitted by ²⁶Mg⁺ is observed, and the equilibrium isconfirmed by the lack of temporal variations of the fluorescenceintensity. The laser-cooling beam conditions are set for the ²⁴Mg⁺laser-cooled ions, so that strong laser-cooling with a large detuningfrequency, typically larger than the natural linewidth, is realized forstable cooling. Next, a probe light 4 with a detuning frequency muchsmaller than the natural linewidth, or substantially 0 MHz, is radiatedon the ²⁶Mg⁺ ions.

[0108] After above operation, mass-spectrometry analysis is performed.The analysis voltage 7 is applied with a frequency sweep, the chopper 5turns the probe beam 4 on and off, and the fluorescence intensityemitted by the probe ions is monitored when the probe light 4 is on.

[0109] Laser parameters for performing analysis are selected aspreviously explained, but a detailed example will be described here. Inthe embodiment of the prior art, a laser-cooled fluorescence massspectrum was obtained when ²⁴Mg⁺ ions were laser-cooled at a detuningfrequency of −50 MHz and with a laser intensity of 100μ watts, in whichthe ion temperature was measured to be 0.38 K prior to analysis. In asaturated state with a laser intensity of 100μ watts, FIG. 6 and theupper diagram of FIG. 22 show that allowable detuning frequencies forstably obtaining 0.38 K are −50 MHz and −300 MHz, as calculatedfollowing the approximation already explained. At −50 MHz, the maximumattainable ion temperature (shown by white circles in FIG. 6 and FIG.22) is 1 K, but at −300 MHz, the maximum attainable ion temperature isapproximately 10 Kelvin. However, if one uses a probe light for ²⁶Mg⁺excitation with 100μ watts intensity and a detuning frequency ofsubstantially 0 MHz, then the ion temperature is calculated to reach 4Kelvin, as shown in the lower figure of FIG. 22, resulting in afluorescence intensity decrease of 50 percent compared to thefluorescence at the initial temperature of 0.38 Kelvin. According toFIG. 22, at a detuning frequency of −50 MHz, this temperature exceedsthe maximum attainable ion temperature 1 Kelvin, so that it is notachieved stably. At the detuning frequency of −300 MHz, this can bestably achieved, and further, there is an ample margin up to the maximumattainable temperature of 10 Kelvin. An ion temperature of 4 Kelvin atthe time of analysis heating could be achieved by applying approximately3 mV as the analysis voltage amplitude V_(ac). By selecting the analysisparameters as described in this method, it is possible to realize morestable mass analysis where ions are less likely to be lost than in themethod of the prior art.

[0110] Embodiment II

[0111] An embodiment is next described utilizing barium ions, Ba⁺, asthe laser-cooled ions in the method previously described in Method (2),where a laser-cooling light and another probe light with differentwavelengths are radiated onto one species of laser-cooled ion. In theexplanation of the embodiment here in particular, the laser-coolingtransition and the probe transition are set the same, and modulation ofthe laser-cooling light intensity is performed utilizing an opticalchopper to avoid the effects of saturation on the probe light.

[0112] The structure of the apparatus is shown in FIG. 3. The followingpoints differ from the first embodiment. Two-color laser system isrequired for laser-cooling of barium ion Ba⁺. An optical chopper 57 isinstalled to modulate the laser cooled light 52. No other species ofions are required for the probe. To acquire probe light, a portion ofthe laser-cooling light is put into an acousto-optical modulator 53, andthe shifted frequency is used as the probe light. The apparatus isequivalent to the apparatus of the first embodiment, whose briefdescription follows. Reference numeral 58 is an ion trap means, 59 is amass analysis power supply, 61 is a photomultiplier tube, 62 is alock-in detector for photons, 63 designates laser cooling ions andsample ions. Each above mentioned components is respectively the same asthe ion trap means 6, the mass analysis power supply 7, thephotomultiplier tube 8, the lock-in detector for photons 9, and thelaser cooled ions and sample ions 10. The reference numeral 60 heredenotes an optical filter. This optical filter 60 is for eliminatingnoise photons caused by light emissions from the laser cooling ions andsample ions 63 due to the pumping-back laser light 56.

[0113] A two-color laser system is required for laser-cooling of thebarium ions Ba⁺. One laser light is called laser-cooling light 52 at awavelength of 493 nm and supplied from a laser light source 52. Theother laser light is called the pumping-back light 56 at a wavelength of650 nm supplied from the laser light source 55, which avoids effects ofoptical pumping to the quasi-stable level of Ba⁺ ion. Sinceimplementation of the laser-cooling of barium ions Ba⁺ has already beenpublished in papers on a large number of research studies in the knownart, explanation is omitted here. Though dye lasers can be used forlaser cooling of the barium ions Ba⁺, semiconductor lasers allow lasercooling of barium ions Ba⁺ more simply and at a lower cost (Reference2).

[0114] In this embodiment, a portion of the laser cooling light 52 isfrequency-shifted and used as the probe light 54. The amount offrequency shift is adjusted so that the frequency-shifted probe light 54attains a detuning frequency of substantially zero MHz, that is, muchless than the natural linewidth. To achieve this, the amount offrequency shift is determined in reference to the optimal detuningfrequency of the laser-cooling light. Since the laser cooling transitionand the probe transition are the same, the probe fluorescence is subjectto saturation effects due to intense laser-cooling light. Since thesaturation effect lowers the sensitivity, the laser light 52 ismodulated by the optical chopper 57, so that the fluorescence from theprobe light is observed only when the laser-cooling light 52 is blocked.

[0115] The selection of parameters when performing analysis isdetermined as previously described. In the embodiment of the prior artthat utilized barium ions Ba⁺, a mass spectrum was acquired at a laserintensity of 50μ watts and at a detuning frequency of −10 MHz. FIG. 23shows the laser-cooling characteristics for this intensity, which resultis not included in FIG. 14 through FIG. 17. At a typical value of 0.38 Kfor the pre-analysis ion temperature, the laser cooling rate at −10 MHzdetuning can be as well achieved at a detuning frequency of −250 MHz asshown in the upper figure of FIG. 23. When these parameters are selectedfor the laser light 52, the maximum ion temperature is about 100 K. Forthe probe beam 54, on the other hand, the detuning frequency is set tosubstantially 0 MHz, and the laser intensity is set to 1μ watt, so thatthe transition will not saturate, while maximizing the fluorescenceintensity. The temperature at which the fluorescence intensity decreases50 percent with respect to a pre-analysis temperature of 0.38 K iscalculated to be 8 K as shown in FIG. 18 and in the lower figure of FIG.23. At −10 MHz detuning, there is a high probability of losing ions,since 1.3 K is almost equal to the maximum ion temperature duringanalysis. At a detuning frequency of −250 MHz, there is sufficientmargin till reaching the maximum ion temperature, so that theprobability of losing ions is greatly decreased. Applying an amplitudeof 0.2 mV as the analysis voltage V_(ac) will achieve an ion temperatureof 8 K during analysis.

[0116] Since the operation procedure for mass analysis is identical tothe procedure of the first embodiment, description is omitted here.

[0117] This invention as described above is therefore capable ofimproved laser-cooled fluorescence mass spectrometry.

What is claimed is:
 1. A laser-cooled fluorescence mass spectrometryapparatus comprising: an ion trap for trapping sample ions, laser-cooledions, and probe ions therein, the probe ions being different ions thanthe laser-cooled ions; first irradiating means for irradiating thesample ions, the laser-cooled ions, and the probe ions in the ion trapwith a first laser beam for cooling the ions; second irradiating meansfor irradiating the sample ions, the laser-cooled ions, and the probeions in the ion trap with a second laser beam for detecting temperaturechanges in the ions; detecting means for detecting the temperaturechanges in the ions; a first ion source for the sample ions; a secondion source for the laser-cooled ions; and a third ion source for theprobe ions.
 2. A laser-cooled fluorescence mass spectrometry apparatusaccording to claim 1, wherein the detuning frequency of the laser beamwhich laser-cools the laser-cooled ions is set to a negative value, andthe absolute value is larger than 100 MHz.
 3. A laser-cooledfluorescence mass spectrometry apparatus according to claim 1, whereinthe intensity of the laser beam which laser-cools the laser-cooled ionsis set to a value so that the spectral width at the Rabi frequencybecomes as large as or larger than the natural linewidth of the coolingtransition.
 4. A laser-cooled fluorescence mass spectrometry apparatusaccording to claim 1, wherein the detuning frequency of the laser beamwhich excites fluorescence of the probe ions is set to an absolute valuesmaller than 10 MHz.
 5. A laser-cooled fluorescence mass spectrometryapparatus according to claim 1, wherein the intensity of the laser beamwhich excites fluorescence of the probe ions is set to a value so thatthe spectral width at the Rabi frequency becomes smaller than thenatural linewidth of the probe transition.
 6. A laser-cooledfluorescence mass spectrometry apparatus according to claim 1, whereinbeam intensity modulation is applied to the second light beam, and thechange in fluorescence of the probe ions is measured as the intensitymodulation of the second laser beam.
 7. A laser-cooled fluorescence massspectrometry apparatus comprising: an ion trap for trapping sample ions,laser-cooled ions, and probe ions therein, the probe ions being the sameions as the laser-cooled ions; first irradiating means for irradiatingthe sample ions, the laser-cooled ions, and the probe ions in the iontrap with a first laser beam for cooling the ions; second irradiatingmeans for irradiating the sample ions, the laser-cooled ions, and theprobe ions in the laser trap with a second laser beam for detectingtemperature changes in the ions; detecting means for detecting thetemperature changes in the ions; a first ion source for the sample ions;a second ion source for the laser-cooled ions; and a third ion sourcefor the probe ions.
 8. A laser-cooled fluorescence mass spectrometryapparatus according to claim 7, wherein the detuning frequency of thelaser beam which laser-cools the laser-cooled ions is set to a negativevalue, and the absolute value is larger than 100 MHz.
 9. A laser-cooledfluorescence mass spectrometry apparatus according to claim 7, whereinthe intensity of the laser beam which laser-cools the laser-cooled ionsis set to a value so that the spectral width at the Rabi frequencybecomes as large as or larger than the natural linewidth of the coolingtransition.
 10. A laser-cooled fluorescence mass spectrometry apparatusaccording to claim 7, wherein the detuning frequency of the laser beamwhich excites fluorescence of the probe ions is set to an absolute valuesmaller than 10 MHz.
 11. A laser-cooled fluorescence mass spectrometryapparatus according to claim 7, wherein the intensity of the laser beamwhich excites fluorescence of the probe ions is set to a value so thatthe spectral width at the Rabi frequency becomes smaller than thenatural linewidth of the probe transition.
 12. A laser-cooledfluorescence mass spectrometry apparatus according to claim 7, whereinbeam intensity modulation is applied to the laser-cooling beam, and thechange in fluorescence of the probe ions is measured when the intensityof the laser-cooling beam is decreased by the modulation.